On Sat, 2022-05-21 at 19:11 +0000, Chuck Lever III wrote:
On May 21, 2022, at 2:10 PM, Trond Myklebust trondmy@hammerspace.com wrote:
On Sat, 2022-05-21 at 17:22 +0000, Chuck Lever III wrote:
On May 20, 2022, at 7:43 PM, Chuck Lever III chuck.lever@oracle.com wrote:
On May 20, 2022, at 6:24 PM, Trond Myklebust trondmy@hammerspace.com wrote:
On Fri, 2022-05-20 at 21:52 +0000, Chuck Lever III wrote:
> On May 20, 2022, at 12:40 PM, Trond Myklebust > trondmy@hammerspace.com wrote: > > On Fri, 2022-05-20 at 15:36 +0000, Chuck Lever III wrote: > > > > > > > On May 11, 2022, at 10:36 AM, Chuck Lever III > > > chuck.lever@oracle.com wrote: > > > > > > > > > > > > > On May 11, 2022, at 10:23 AM, Greg KH > > > > gregkh@linuxfoundation.org wrote: > > > > > > > > On Wed, May 11, 2022 at 02:16:19PM +0000, Chuck > > > > Lever > > > > III > > > > wrote: > > > > > > > > > > > > > > > > On May 11, 2022, at 8:38 AM, Greg KH > > > > > > gregkh@linuxfoundation.org wrote: > > > > > > > > > > > > On Wed, May 11, 2022 at 12:03:13PM +0200, > > > > > > Wolfgang > > > > > > Walter > > > > > > wrote: > > > > > > > Hi, > > > > > > > > > > > > > > starting with 5.4.188 wie see a massive > > > > > > > performance > > > > > > > regression on our > > > > > > > nfs-server. It basically is serving requests > > > > > > > very > > > > > > > very > > > > > > > slowly with cpu > > > > > > > utilization of 100% (with 5.4.187 and earlier > > > > > > > it > > > > > > > is > > > > > > > 10%) so > > > > > > > that it is > > > > > > > unusable as a fileserver. > > > > > > > > > > > > > > The culprit are commits (or one of it): > > > > > > > > > > > > > > c32f1041382a88b17da5736886da4a492353a1bb > > > > > > > "nfsd: > > > > > > > cleanup > > > > > > > nfsd_file_lru_dispose()" > > > > > > > 628adfa21815f74c04724abc85847f24b5dd1645 > > > > > > > "nfsd: > > > > > > > Containerise filecache > > > > > > > laundrette" > > > > > > > > > > > > > > (upstream > > > > > > > 36ebbdb96b694dd9c6b25ad98f2bbd263d022b63 and > > > > > > > 9542e6a643fc69d528dfb3303f145719c61d3050) > > > > > > > > > > > > > > If I revert them in v5.4.192 the kernel works > > > > > > > as > > > > > > > before > > > > > > > and > > > > > > > performance is > > > > > > > ok again. > > > > > > > > > > > > > > I did not try to revert them one by one as > > > > > > > any > > > > > > > disruption > > > > > > > of our nfs-server > > > > > > > is a severe problem for us and I'm not sure > > > > > > > if > > > > > > > they are > > > > > > > related. > > > > > > > > > > > > > > 5.10 and 5.15 both always performed very > > > > > > > badly on > > > > > > > our > > > > > > > nfs- > > > > > > > server in a > > > > > > > similar way so we were stuck with 5.4. > > > > > > > > > > > > > > I now think this is because of > > > > > > > 36ebbdb96b694dd9c6b25ad98f2bbd263d022b63 > > > > > > > and/or > > > > > > > 9542e6a643fc69d528dfb3303f145719c61d3050 > > > > > > > though > > > > > > > I > > > > > > > didn't tried to > > > > > > > revert them in 5.15 yet. > > > > > > > > > > > > Odds are 5.18-rc6 is also a problem? > > > > > > > > > > We believe that > > > > > > > > > > 6b8a94332ee4 ("nfsd: Fix a write performance > > > > > regression") > > > > > > > > > > addresses the performance regression. It was > > > > > merged > > > > > into > > > > > 5.18- > > > > > rc. > > > > > > > > And into 5.17.4 if someone wants to try that > > > > release. > > > > > > I don't have a lot of time to backport this one > > > myself, > > > so > > > I welcome anyone who wants to apply that commit to > > > their > > > favorite LTS kernel and test it for us. > > > > > > > > > > > > If so, I'll just wait for the fix to get into > > > > > > Linus's > > > > > > tree as > > > > > > this does > > > > > > not seem to be a stable-tree-only issue. > > > > > > > > > > Unfortunately I've received a recent report that > > > > > the > > > > > fix > > > > > introduces > > > > > a "sleep while spinlock is held" for NFSv4.0 in > > > > > rare > > > > > cases. > > > > > > > > Ick, not good, any potential fixes for that? > > > > > > Not yet. I was at LSF last week, so I've just started > > > digging > > > into this one. I've confirmed that the report is a > > > real > > > bug, > > > but we still don't know how hard it is to hit it with > > > real > > > workloads. > > > > We believe the following, which should be part of the > > first > > NFSD pull request for 5.19, will properly address the > > splat. > > > > https://git.kernel.org/pub/scm/linux/kernel/git/cel/linux.git/commit/?h=for-... > > > > > Uh... What happens if you have 2 simultaneous calls to > nfsd4_release_lockowner() for the same file? i.e. 2 > separate > processes > owned by the same user, both locking the same file. > > Can't that cause the 'putlist' to get corrupted when both > callers > add > the same nf->nf_putfile to two separate lists?
IIUC, cl_lock serializes the two RELEASE_LOCKOWNER calls.
The first call finds the lockowner in cl_ownerstr_hashtbl and unhashes it before releasing cl_lock.
Then the second cannot find that lockowner, thus it can't requeue it for bulk_put.
Am I missing something?
In the example I quoted, there are 2 separate processes running on the client. Those processes could share the same open owner + open stateid, and hence the same struct nfs4_file, since that depends only on the process credentials matching. However they will not normally share a lock owner, since POSIX does not expect different processes to share locks.
IOW: The point is that one can relatively easily create 2 different lock owners with different lock stateids that share the same underlying struct nfs4_file.
Is there a similar exposure if two different clients are locking the same file? If so, then we can't use a per-nfs4_client semaphore to serialize access to the nf_putfile field.
I had a thought about an alternate approach.
Create a second nfsd_file_put API that is not allowed to sleep. Let's call it "nfsd_file_put_async()". Teach check_for_locked() to use that instead of nfsd_file_put().
Here's where I'm a little fuzzy: nfsd_file_put_async() could do something like:
void nfsd_file_put_async(struct nfsd_file *nf) { if (refcount_dec_and_test(&nf->nf_ref)) nfsd_file_close_inode(nf->nf_inode); }
That approach moves the sync to the garbage collector, which was exactly what we're trying to avoid in the first place.
Totally understood.
My thought was that "put" for RELEASE_LOCKOWNER/FREE_STATEID would be unlikely to have any data to sync -- callers that actually have data to flush are elsewhere, and those would continue to use the synchronous nfsd_file_put() API.
Do you have a workload where we can test this assumption?
Why not just do this "check_for_locks()" thing differently?
It really shouldn't be too hard to add something to nfsd4_lm_get_owner()/nfsd4_lm_put_owner() that bumps a counter in the lockowner in order to tell you whether or not locks are still held instead of doing this bone headed walk through the list of locks.
I thought of that a couple weeks ago. That doesn't work because you can lock or unlock by range. That means the symmetry of LOCK and LOCKU is not guaranteed, and I don't believe these calls are used that way anyway. So I abandoned the idea of using get_owner / put_owner.
Then you're misunderstanding how it works. lm_get_owner() is called when a lock is initialised from another one. The whole point is to ensure that each and every object representing a range lock on the inode's list maintains its own private reference to the knfsd lockowner (i.e. the fl->fl_owner).
For instance when a LOCK call calls posix_lock_inode(), then that function uses locks_copy_conflock() (which calls lm_get_owner) to initialise the range lock object that is being put on the inode list. If the new lock causes multiple existing locks to be replaced, they all call lm_put_owner to release their references to fl->fl_owner as part of the process of being freed.
Conversely, when LOCKU causes a range to get split, the two locks that replace the old one are both initialised using locks_copy_conflock(), so they both call lm_get_owner. The lock that represents the range being replaced is then made to call lm_put_owner() when it is freed.
etc, etc...
But maybe we can provide some other mechanism to record whether a lockowner is associated with file locks.